Immediate dropping of outgoing calls on Galaxy Note 4

The first of my three problems. Should mention that this is my first smart phone. In general, my level of tech comprehension and educability is probably a bit above average for a civilian.

So here's the problem. If I dial a phone number, or call directly from my contacts, it works sometimes. On other occasions, when I press dial, I get the green-colored screen showing that the number is being dialed. Before it rings, however, it switches to the red-colored screen that says "call ended." The people I've tried to call haven't reported that their phones have rung.

If I retry, the problem recurs. If I try by a different method, e.g., dialing the number rather than calling from contacts, the problem recurs. If I try calling someone else, such as my landline, the problem recurs. In other words, I can't call out at all.

On other occasions, however, it works fine. I have not noted anything systematic to differentiate the circumstances in which it works and doesn't work.


Hardware & other details:

1. Galaxy Note 4 model SM-N910F
2. Android version 5.0.1.
3. Security software version MDF v1.1 Release 5; VPN v1.4 Release 4.
4. Service from giffgaff, so I'm usually on the O2 network.